Stack molding provides an advantage over single molding in that it enables the output of an injection molding machine to be at least doubled without significantly increasing its size. Stack mold configurations, such as those shown in U.S. Pat. No. 4,891,001 to Gellert, U.S. Pat. No. 5,846,472 to Rozema, U.S. Pat. No. 5,229,145 to Brown, and U.S. Pat. No. 7,115,226 to Olaru, each of which is incorporated by reference herein in its entirety, generally employ a stationary first platen, a movable center platen and a movable second platen. The mold cavities are conventionally located on opposing faces of the movable center platen. The movable center platen and the second movable platen reciprocate to open and close the mold cavities during a production cycle.
In a stack molding apparatus, a manifold system extends through the center platen in order to reach the mold cavities located on each side of the center platen via branching manifold melt channels. In some instances, multi-cavity stack molds use a valve-gated melt transfer nozzle, which is coupled to the stationary platen, to deliver a melt from an extruder nozzle of the injection molding machine to a second valve-gated melt transfer nozzle, which is coupled to the movable center platen, to transfer the melt to the manifold. The manifold than delivers melt into various hot runner injection molding nozzles that are associated with each individual mold cavity. In a variation on these arrangements, the melt transfer nozzles may instead be thermal gated.
In conventional stack molding arrangements using nozzle to nozzle melt transfer, heat expansion contributes to proper sealing between the two nozzles, which results in a very sensitive solution where processing temperatures influence the performance of the seal. For instance, if a lower processing temperature is used it may reduce the heat expansion of one or both of the melt transfer nozzles thereby adversely affecting the sealing contact there between. One approach to addressing the problem of leakage due to improper sealing is to calculate the heat expansion of the melt transfer nozzles and determine a preload that is to be maintained between the two nozzles in the mold closed configuration. However, a lack of proper preload and/or a lower processing temperature often creates a gap between the two melt transfer nozzles where molten plastic gets trapped, thereby resulting in “leakage.” At the other extreme, an excessive preload may constrain the melt transfer nozzle assemblies, which can cause damage to these components and/or result in some “leakage” at the transfer point between the downstream melt transfer nozzle and the manifold. As such, a stack molding arrangement where sealing between the melt transfer nozzles is not dependent on the heat expansion of the nozzles may provide a desirable solution for certain injection molding applications.